An overspeed governor is normally used for monitoring unintended movement of an elevator car. The overspeed governor activates a safety gear preventing movement of the elevator car, if the speed grows of the elevator car becomes too high. The overspeed governor is connected to the safety gear with a rope, which passes via the rope pulley of the overspeed governor. The rope pulley of the overspeed governor is normally able to rotate freely when the elevator car is moving. The overspeed governor activates the safety gear by stopping the movement of the rope of the safety gear. In practice, this occurs by locking the movement of the rope pulley of the overspeed governor with a locking means of the rope pulley, if the speed of the elevator car becomes too high. The locking means shifts from a position permitting movement of the rope pulley into a position preventing movement of the rope pulley from the effect of centrifugal force.
An electronic overspeed governor is also proposed for monitoring unintended movement of an elevator car. An electronic overspeed governor comprises a microprocessor control, so that it enables more versatile monitoring of the movement of an elevator car than before. A number of values can be set for the limit value for the maximum permitted speed of the elevator car and the values can also be changed as a function of the position of the elevator car e.g. such that the limit value for the maximum permitted speed decreases when the elevator car approaches the end of the elevator hoistway. Publication U.S. Pat. No. 6,170,614 B1 presents the operating principle of one electronic overspeed governor.
Despite their obvious advantages electronic overspeed governors have not yet, however, displaced conventional mechanically-controlled overspeed governors to any significant extent in the safety arrangements of elevators. Conventional mechanically-controlled overspeed governors have retained their position owing to inter alia their simplicity, operational reliability and reliable structure.
An electronic overspeed governor also functions as an elevator safety device that is required by elevator regulations. For this reason an overspeed governor must be designed to be fail-safe such that a malfunction in the overspeed governor, e.g. a disturbance of the electricity supply of the overspeed governor, always results in the gripping of a moving elevator car.
One problem related to an electronic overspeed governor is how to ensure the operation of the overspeed governor in connection with an electricity outage. An elevator car must be able to be moved e.g. in an emergency braking situation and/or in an emergency rescue situation regardless of an electricity outage. Earlier this problem has been solved by using an accumulator of sufficiently large charge capacity as a reserve power source of the overspeed governor, which accumulator supplies current to the overspeed governor during an electricity outage. A drawback in this solution is the unpredictability of the timing of the emergency rescue, especially if the electricity outage affects a large city or large part of a city. If the accumulators have emptied there is nothing to indicate the operating condition of the electronic overspeed governor to the rescue personnel. In this case, when trying to move the elevator car by opening the machinery brakes the safety gear stops any movement of the elevator car.